These studies are aimed at investigating basic mechanisms of cell damage by hyperoxia with special interest and emphasis on red cell lysis. Although the toxic effect of molecular oxygen on the tissues has been recognized for many years, the current use of 100% oxygen environment in space capsules and the use of oxygen under high pressure for medical surgical purposes has made these effects of considerably greater practical importance. To the present time, all studies have been limited to clinical and histo-pathologic considerations. We are interested in identifying the basic processes leading to cell damage which are associated with stress related to chemical and physical oxidants including molecular oxygen, ultraviolet radiation, radiation, certain drugs and to eventually extend these observations to their possible applicability in the treatment of neoplastic disease. Studies of carbohydrate metabolism and oxido-reduction transformation systems are of interest in these cells. Recently studies have led us to an investigation of paroxysmal nocturnal hemoglobinuria with a more detailed analysis of lipid and protein interaction in these cell membranes.